Phenol-aldehyde resins are good adhesives for wood products. Exemplary wood products include plywood, particle board, veneers, oriented strandboard, hard board, and waferboard type wood products. These resins exhibit high strength and good chemical resistance making them desirable for wood products facing harsh environmental conditions. Unfortunately, resins do not become activated until 55.degree. C. and require relatively long periods to set at room temperature as well as temperatures over 200.degree. F. as are routinely used for making wood products.
An ideal resin adhesive would have a low viscosity and allow sufficient time for manipulation of the coated product. The time for manipulation is generally referred to as the "assembly time" or the "pot life." The time needed for applying the resin adhesive and manipulating the wood product will depend on the particular wood product.
Thereafter, the adhesive viscosity should increase quickly, become set, and cure in a minimum amount of time. Cure accelerating agents (also known as "hardening agents") are typically added to the resin at some stage in the process to adjust the hardening rate.
Isocyanate and alkylene carbonate curing agents are recognized as effective hardening agents but each has certain drawbacks. Isocyanate poses safety and environmental problems. Alkylene carbonates are too fast. They induce substantially instantaneous crosslinking and have a negligible pot life. Propylene carbonate gels instantaneously at room temperature and in about 30 seconds at 0.degree. F. This pot life is far too short for wood product manipulation particularly at the far higher temperatures normally encountered in commercial process. Most continuous plants require 20-45 minutes for assembly.
It would be desirable to have a curing agent that could be premixed with the resin and retain a low viscosity for assembly of the wood but which would set and cure quickly at economically low temperatures.